In power system protection devices, potential transformers (PTs) and current transformers (CTs) have traditionally been used to reduce power line voltages and currents from their very high line levels to specific, relatively low levels, i.e. 70 volts and 5 amps. These signals are then applied to protective relays which determine power line fault conditions. As a consequence of a fault condition, current on the power line will increase such that the output from the CT increases to between 6 and 100 amps from the normal 5 amps. With conventional electromechanical relays, the output signals from PTs and CTs are applied to the relays directly; operating power for those relays is also provided by the PTs and CTs.
With the introduction of solid state, and then computer-type protective relays, the outputs of the conventional PTs and CTs (referred to herein as instrument Pts and CTs) were far too high to be compatible with the electronic circuitry of those relays, and, therefore, solid state/computer protective relays include input transformers, and voltage dividers and/or shunts, which reduce the signal levels from the PTs and CTs down to, for instance, 1.4 volts and 100 MA, which are appropriate for electronic circuits.
To carry out necessary testing of electromechanical relays, test units were designed which are capable of reproducing the non-fault outputs of the conventional PTs and CTs, as well as reproducing the PT/CT outputs under certain line fault conditions. Ideally, this requires a test unit capable of producing 1000 VA (which is accomplished by injecting 100 amperes into an impedance of 0.1 ohms). As a practical matter, such test units will typically produce a test current of 25 amperes, producing 62.5 VA. This compromise, however, still requires large amplifiers and power supplies, which are heavy, bulky and expensive.
Such test units continue to be used even though many protective relays are now solid state/computer type relays which in operation do not require the conventional PT/CT signal levels, as indicated above. It would hence be desirable to have a test unit which produces a 1 VA output signal, which would be adequate for the relay circuits per se, in order to minimize weight and expense of the test unit. Also, it would be desirable to be able to test separately the operation of the input transformers and the operation of the actual relay itself under simulated fault conditions.